The present invention relates to an electrostatic image developer used for developing an electrostatic latent image formed on the surface of a latent image carrier body in an electrophotographic method, an electrostatic recording method, and an electrostatic printing method, or the like. More particularly, this invention relates to the electrostatic image developer having a toner and a carrier.
In association with social movement of energy saving in recent years, such movement has become active also in the electrophotographic field. A fixing part in particular of electrophotographic equipment consumes a large amount of energy. Therefore, studies in this part have been carried out so as to attain mechanically low power consumption. Regarding toner as a supply, development of toner adequate for an energy-saving fixing device has been urged as well. As measures directed toward achievement of energy saving in the fixing device due to toner, the followings are most popular.
For example, some of binder resins for toner is targeted to stabilize fixability at a low temperature by using a resin having low molecular weight. Although energy saving can be achieved because of low-temperature fixability of toner, the toner itself or toner components may be melted in many cases on the surface of a carrier (hereafter referred to as toner spent) particularly when the binder resin of the toner has a large amount of components having low molecular weight. The carrier surface is soiled with this toner spent, and the charged sites of the carrier decrease, which causes the amount of triboelectric charge as a two-component developer to vary. Resultantly, inconvenience such as occurrence of variation in image density or fog may occur.
In Japanese Patent Application Laid-Open No. 10-198068, detailed description on toner molecular weight for suppression of toner spent has been disclosed. However, low-energy consumption for the fixing device could not sufficiently be achieved with the range of the toner molecular weight in this publication.
Further, as image forming devices such as electrophotographic copiers have been widely used, the purpose of their use also becomes widely, demands of the market for high-definition and high-quality images are increasing. In this technological field, such attempt that toner particle diameter is made smaller to achieve higher quality of image has been made. However, smaller particle diameter makes the surface area per unit weight increase, and the amount of electrification on toner tends to increase. Therefore, lower image density and deterioration of durability are what may be concerned about. In addition, because of the large amount of electrification on toner, adhesion of toner particles is strong and fluidity is lowered. Accordingly, there comes up some problems in stability of toner supply and impartation of triboelectric charge to toner to be supplied. In general, the tendency of increase in the amount of electrification becomes more significant particularly when polyester-based binder of high chargeability is used.
Some types of developers have been proposed for obtaining improved image quality. In Japanese Patent Application Laid-Open No. 51-3244, a nonmagnetic toner for improving image quality obtained by restricting a particle size distribution has been proposed. The toner contains mainly toner particles having particle diameter of 8 to 12 xcexcm, which are comparatively coarse. Therefore, according to the analysis of the inventors, uniform xe2x80x9cdepositionxe2x80x9d of the toner onto a latent image is difficult with this particle size. Further, the toner has properties such that the particles of 5 xcexcm or less are contained at 30 number % or less, and the particles of 20 xcexcm or more are contained at 5 number % or less. Therefore, the particle size distribution is broad, which also tends to decrease the degree of uniformity. In order to form a sharp image using the toner having such coarse toner particles and broad particle size distribution, it is required to increase the apparent image density by embedding space of inter-toner particles through heavy superimposition of the particles on each other. Resultantly, this invention also has a problem such that the amount of toner consumption required for attaining the predetermined image density increases.
Japanese Patent Application Laid-Open No. 54-72054 has made the proposal of a nonmagnetic toner having particle size distribution sharper than the former one. The size of particles having an intermediate weight is as coarse as a range of 8.5 to 11.0 xcexcm. Therefore, this toner is still in need of some improvements as high-resolution toner. Further, Japanese Patent Application Laid-Open No. 58-129437 has made the proposal of a nonmagnetic toner having 6 to 10 xcexcm average particle diameter and 5 to 8 xcexcm diameter in most frequency particles. However, the toner contains a small amount of particles of 5 xcexcm or less at 15 number % or less, which means this invention has little effect on a sharp image.
In accordance with studies by the inventors, it is found that the toner particles of 5 xcexcm or less sharply reproduce the edge of a latent image and have a main function for solid xe2x80x9cdepositionxe2x80x9d of the toner onto the entire latent image. Particularly, in the electrostatic latent image on a photoreceptor, the edge part as its outline has electric field strength stronger than its internal part because the lines of electric force are concentrated on the edge part. Therefore, sharpness of image quality is determined depending on the quality of the toner particles collecting on this part. According to the inventors, it is found that the amount of the particles of 5 xcexcm or less is effective in solution of the problem on sharpness of image quality. In Japanese Patent Application Laid-Open No. 2-222966, a toner, that contains toner particles having particle diameter of 5 xcexcm or less at 15 to 40 number %, has been proposed. It is thought that this invention has achieved significantly improved image quality. However, further improved image quality is desired.
In Japanese Patent Application Laid-Open No. 2-877, a toner, that contains toner particles having particle diameter of 5 xcexcm or less at 17 to 60 number %, has been proposed. The image quality and image density are surely stabilized with this toner. However, it is also found that it is difficult to constantly obtain a predetermined level of image quality. That is because the particle size distribution of the toner varies when an original like a photographic document, that requires a large amount of toner consumption, is repeatedly printed. Further, all the above-mentioned inventions relate only to a nonmagnetic toner developer, with which high image quality has been obtained in terms of reproducibility of fine lines or the like, but measures against background dirt or the like have not been improved yet.
On the other hand, inventions, which suggest average particle diameter and particle size distribution of carrier, have been disclosed in Japanese Patent Application Laid-Open No. 51-3238, Japanese Patent Application Laid-Open No. 58-144839, and Japanese Patent Application Laid-Open No. 61-204646. Japanese Patent Application Laid-Open No. 51-3238 mentions the coarse particle distribution.
However, this invention does not particularly disclose the magnetic property having a close relation with developing performance of the developer or conveyability in the developing device. Further, the carrier in the example contains particles of 250 mesh or more at about 80 weight % or more based on the total weight of the carrier, and its average particle diameter is 60 xcexcm or more.
Japanese Patent Application Laid-Open No. 58-144839 has disclosed merely the average particle diameter, and neither mentions the amount of fine powder that exerts an effect on carrier adhesion to a photoreceptor and also the amount of coarse powder that exerts an effect on sharpness of an image, nor describes the distribution of the powder in detail. Further, the invention of Japanese Patent Application Laid-Open No. 61-204646 has disclosed a combination of a copier and an appropriate developer as its essence, but has not particularly described particle size distribution and magnetic properties of the carrier. The invention does not even disclose why this developer is effective to the copier. The ferrite carrier disclosed in Japanese Patent Application Laid-Open No. 58-23032 is based on a porous material that has many voids, and such carrier makes an edge effect easily occur and has insufficient durability.
Such a developer as follows has been long awaited. The developer is one with which continuous duplication of an image having a large area can be carried out using a small amount of the developer, and which can satisfy the property that the edge effect does not occur after the durability passes. The studies in the developer and the carrier have been still continued, and much-awaited carrier is as follows. This carrier has the ability to continuously copy an image having an image area of 20% or more that is almost a solid image, and also the abilities to reduce the edge effect and keep evenness of image density in a sheet of copied matter.
Japanese Patent Application Laid-Open No. 02-281280 has made the proposal of a carrier having a narrow particle size distribution with the controlled amount of existence of fine powder and amount of existence of coarse powder. The carrier with the improved developing property has been achieved in the above-mentioned invention.
However, as mentioned above, the demands of the market for higher definition and higher quality images of copiers are increasing, and in this technical field, an attempt to achieve a higher degree of image quality has been made by reducing the particle diameter of toner. However, there comes up a problem that a surface area per unit weight increases as the particle diameter becomes finer and the amount of electrification on toner tends to increase, which may cause the image density to be lowered and durability to be deteriorated.
As explained above, prevention of the image density from being lowered and the durability from deterioration due to finer toner particle diameter, or further finer diameter of carrier for improving developing efficiency has been studied. However, the current situation indicates that such carrier has not quality sufficient enough to follow variations in the charge amount due to improved durability.
The inventors have found the facts as follows after carefully studying image density, reproducibility of a high light, and reproducibility of fine lines in the image forming method. That is, higher degree of image quality excellent in high degree of image density, reproducibility of a highlight and reproducibility of fine lines, etc. can be achieved when both a toner having a specific particle size distribution and a spherical carrier are used. Further, when specific titania particles as an external additive are contained in the toner, improved developer fluidity and stable environmental characteristic can be achieved.
Disadvantages, that occur when respective particle diameter of the carrier and the toner is made smaller, include cases that fluidity as a developer is lowered and the developer in a developing device is hard to be circulated. For the countermeasures against these troubles, the condition of the device may be changed such that stirring strength in the developing device is enhanced. However, here occurs a problem such that the change of the device condition makes the durability of the developer shortened. Therefore, this change is not preferable. To solve the problem, it is important to keep a predetermined level of fluidity as a developer. Some means are conceivable in order to keep fluidity of the developer.
As one of the means, the inventors have found that controlling a shape of carrier is effective. That is, increasing the degree of sphericity of carrier particles makes the fluidity improved.
The sphericity of a carrier has been described in Japanese Patent Application Laid-Open No. 59-222847. However, definition of the degree of sphericity is not clear, so that it is impossible to know which level of the sphericity is actually available.
In Japanese Patent Application Laid-Open No. 63-41864, the sphericity value xcexa8Z of a carrier is defined, but this definition works only in a developing method based on coating by an elastic blade, which is different from the present invention.
The inventors have found the fact as follows. That is, in order to improve the fluidity of a developer, it is also effective that resin having low-surface energy is further contained in coating resin for the carrier coated with the resin.
Inventions that define the fluidity of a carrier have been disclosed in Japanese Patent Application Laid-Open No. 63-41865 and Japanese Patent Application Laid-Open No. 01-225962. However, when the carrier in the present invention smaller than the conventional carrier is used, measurement by JIS-Z2502 is difficult, and reproducibility of an image is hard to be obtained.
Further, the publications have restriction only to the carrier, and do not include electrification on the toner side and influence of other additives. Accordingly, even if the fluidity of a carrier is set to a predetermined level, a sufficient result is not always obtained in a practical case. Therefore, it is important to pay attention on the fluidity of developer including electrification and its influence on the surface of toner, or the like.
It is an object of this invention to provide a two-component developer using a low-temperature fixing toner, in which toner spent on the surface of a carrier is presented in a small amount, the sufficient amount of triboelectric charge is imparted, and the triboelectrification is stabilized.
Another object of this invention is to provide a two-component developer which is clear and excellent in gradation.
Still another object of this invention is to provide a two-component developer which is excellent in conveyability in a developing device.
A further object of this invention is to provide a two-component developer whose performance is not changed after a long period of its use.
A still further object of this invention is to provide a two-component developer whose performance is not changed even by environmental variations.
A still further object of this invention is to provide a two-component developer with which high image density can be obtained with a minimum consumption of this developer.
A still further object of this invention is to provide a two-component developer with which a toner image excellent in resolution, gradation, and reproducibility of fine lines can be formed in an image formation apparatus based on digital image signals.
According to this invention, firstly, the two-component developer as follows is provided. This two-component developer consists of at least a toner and a carrier, in which number average molecular weight (Mn) of the toner is 3,000 or less, and molecules having a molecular weight of 1,000 or less are contained at 40 number % or more, and the carrier satisfies general formula (1) as follows:
[2] 3,000,000xe2x89xa6"sgr"1000xc3x97Dc3xe2x89xa620,000,000xe2x80x83xe2x80x83(1) 
where "sgr"1000 represents a magnetization (emu/g) of a carrier at 1,000 oersted, and Dc represents volume average particle diameter (xcexcm) of carrier.
Secondly, the two-component developer according to the first description is provided as follows. In this developer, the volume average particle diameter (Dc) of the carrier is not larger than 60 xcexcm.
Thirdly, the two-component developer according to the first or second description is provided as follows. In this developer, the toner is a magnetic toner, which has weight average particle diameter of 3 to 7 xcexcm, and contains toner having particle diameter of 5.04 xcexcm or less at more than 40 number %, toner having particle diameter of 4 xcexcm or less at 10-70 number %, toner having particle diameter of 8 xcexcm or more at 2 to 20 volume %, and toner having particle diameter of 10.08 xcexcm or more at 6 volume % or less.
Fourthly, the two-component developer according to any of the first to third descriptions is provided as follows. In this developer, the carrier has volume average particle diameter of 15 to 45 xcexcm, and contains carrier particles smaller than 22 xcexcm at 10 to 20%, carrier particles smaller than 16 xcexcm at 3% or less, carrier particles of 62 xcexcm or more at 2 to 15%, and carrier particles of 88 xcexcm or more at 2% or less.
Fifthly, the two-component developer according to any of the first to fourth descriptions is provided as follows. In this developer, a saturation magnetization to an applied magnetic field with 1,000 oersted of the carrier is 40 to 120 emu/g, residual magnetization is not more than 10 emu/g, and a coercive force is not more than 60 oersted.
Sixthly, the two-component developer according to any of the first to fifth descriptions is provided as follows. In this developer, fluidity of the developer is 25 to 55 (sec/50 g).
Seventhly, the two-component developer according to any of the first to sixth descriptions is provided as follows. In this developer, the external additive is titania particles whose average particle diameter is 0.01 to 0.2 xcexcm, hydrophobic degree is 20 to 98%, and light transmittance in 400 nm is not less than 40%.
Eighthly, the two-component developer according to any of the first to seventh descriptions is provided as follows. In this developer, the carrier particle has a shape such that a ratio between its length (X) and breadth (Y) is in a range from 0.6 to 1.0 on the average when the carrier particle is regarded as a plane image.
Ninthly, the container filled with the two-component developer according to any of the first to eighth descriptions is provided.
Tenthly, the image formation apparatus with the built-in container according to the ninth description is provided.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.